1. Field
The disclosed concept pertains generally to electrical switching apparatus and, more particularly, to electrical switching apparatus, such as contactors, starters, motor protection relays and circuit breakers. The disclosed concept also pertains to protection relays.
2. Background Information
A starter is a device used to provide protection and control for a motor, such as a three-phase motor and the like, typically used in, for example, various industrial settings to operate machinery. A common type of starter includes an electromagnetic contactor and a motor protection relay. The contactor includes separable contacts that are connected to each phase of the electric motor, such that, when closed, the separable contacts connect the motor to an electrical source, such as a three-phase electrical source. The motor protection relay monitors the load current and, in some cases, voltage, and trips the contactor open if a problem condition, such as persistent overcurrent, arises. Typically, the motor protection relay tracks an ht characteristic of the load current, which is a measure of heating. Motor protection relays typically include a microprocessor, which digitally generates the ht value. When the calculated I2t value reaches a designated trip level, the contactor is tripped open to interrupt the flow of current to the motor.
U.S. Pat. No. 4,743,818 discloses a multi-function motor protective relay that monitors three-phase alternating current (AC) and makes separate trip and alarm decisions based on pre-programmed motor current and temperature conditions. The temperature of a rotating rotor is inferred from the current flowing in the lines which supply the motor and from resistance temperature detection (RTD) devices in the stator of the motor. A system RTD measures motor stator winding temperatures.
U.S. Pat. No. 5,485,491 discloses an online system for diagnosing operability of a rotating electrical power conversion apparatus. The system includes sensing means for sensing at least one electrical insulation non-thermal parameter of the rotating electrical power conversion apparatus during operation thereof and producing a corresponding at least one electrical variable; data conversion means for converting the at least one electrical variable to a corresponding at least one value; comparison means for comparing the at least one value to a corresponding at least one predetermined baseline value of the rotating electrical power conversion apparatus and producing a corresponding at least one comparison value; and signaling means for outputting at least one signal related to a period of predicted operability of the rotating electrical power conversion apparatus whenever the at least one comparison value exceeds a corresponding at least one predetermined dead band value. Temperature sensors are provided for sensing oil temperature, water inlet temperature, water outlet temperature, oil inlet temperature, and temperature of guide and thrust bearings.
The insulation of a rotating electrical apparatus, such as a motor, has a finite life. Various operating conditions (e.g., without limitation, moisture; temperature; mechanical stress; dust; airborne chemicals), affect the actual time it takes to reach the “end of life” of the motor insulation.